222 THE MIGRATION OF BRITISH BIRDS 



no reasonable amount oS. slow physical change would 

 arrest it. 



We now pass to England. There can be no question 

 that a land connection endured across the mouth of the 

 English Channel after the submergence which took place 

 off the south coast of Ireland, and that which severed 

 Ireland from England. All the evidence suggests that 

 this submergence took place, both east and west of the 

 British Islands, from north to south, as we shall also 

 learn when we come to study the routes across the 

 North Sea. Whilst the mouth of the English Channel 

 remained dry land, a few more of our summer visitors 

 succeeded in extending their range northwards to Eng- 

 land — among these later arrivals we may mention the 

 Redstart and the Tree Pipit, but the Nightingale, the 

 Reed Warbler, and other eastern species could not have 

 done so. The Channel Islands remained continental 

 until the sea had encroached some distance up the 

 English Channel. As the sea gradually encroached upon 

 the land, and the English Channel began to appear, 

 other birds gradually extended their range northwards 

 across what are now the narrower portions of the 

 Channel, and their line of migration was continued 

 across the widening water area year by year, no single 

 generation of birds, of course, being able to perceive 

 the slightest change in their route. Einally, and at a 

 much later period, when the climate must have grown 

 considerably milder, the land mass of the Channel 

 could not have extended much further west of the Isle 

 of Wight ; and across this narrow neck all the latest 

 species to arrive made their final entry before the 

 Avaters of the Strait of Dover mingled with those of 



